1. Field of the Invention
The present invention relates to variable valve control system and method for a (vehicular) multi-cylinder internal combustion engine in which a variably operated valve mechanism which continuously varies a lift characteristic of an intake valve or an exhaust valve is equipped for each of a plurality of cylinder groups and, particularly, relates to a technique of a correction or learning (corrective learning) process of a reference position of the above-described lift characteristic.
2. Description of the Related Art
In an internal combustion engine, particularly, in a gasoline internal combustion engine which develops an output in accordance with an intake air quantity, a variably operated valve mechanism which varies a lift characteristic of an intake valve in accordance with an engine driving condition has been used in order to improve a fuel consumption. A Japanese Patent Application First Publication No. 2002-180856 published on Jun. 26, 2002 exemplifies a previously proposed variable valve control system in which a hydraulic pressure driven variable operated valve mechanism which modifies a rotational phase of a camshaft with respect to a crankshaft to enable a continuous modification of a valve timing is installed. An actual phase angle of the camshaft is detected by means of a crank angle sensor and a cam angle sensor. This Japanese Patent Application First Publication describes that, during a fuel supply cut-off at a time of a vehicle deceleration, the camshaft is placed at a most retarded angle position, the lift characteristic is minimized, a learning process for a sensor output reference position is carried out, and, in this way, under a situation that the fuel supply is stopped, the learning process is carried out so that an increase in the fuel consumption is eliminated and the fuel consumption can be improved.